Methods and systems for multi-frequency transducer array fabrication

ABSTRACT

Various methods and systems are provided for a multi-frequency transducer array. In one example, the transducer array is fabricated by forming an interdigitated structure from a first comb structure with a first sub-element and a second comb structure with a second sub-element. The interdigitated structure is coupled to a base package, a matching layer, and a backing layer to form a plurality of multi-frequency transducers.

FIELD

Embodiments of the subject matter disclosed herein relate to atransducer for a medical device.

BACKGROUND

Transducer probes are used in a variety of applications to convertenergy from a physical form to an electrical form. For example, atransducer probe may include piezoelectric materials which generateelectrical voltage from a mechanical stress or strain exerted on thematerials. Piezoelectric transducer probes are configured to be highlysensitive to provide large signal amplitudes, broad bandwidth for useacross a wide range of frequencies, and short-duration impulse for highaxial resolution. Such properties are desirable for medical applicationssuch as imaging, non-destructive evaluation, fluid flow sensing, etc.Furthermore, frequency apodization of the transducer probe may mitigateloss of signal resolution due to signal attenuation and dispersion asthe signal travels away from its source.

BRIEF DESCRIPTION

In one embodiment, a method includes forming a first comb structure witha first sub-element having a first resonance frequency, forming a secondcomb structure, complementary in geometry to the first comb structurewith a second sub-element having a second resonance frequency, combiningthe first and second comb structures to form an interdigitatedstructure, forming a third acoustic stack by coupling the interdigitatedstructure to a base package, and coupling the third acoustic stack to amatching layer block and a back layer block to form a plurality ofmulti-frequency transducers. In this way, a transducer array may befabricated via an approach that allows a frequency range of thetransducer array to be adjusted.

It should be understood that the brief description above is provided tointroduce in simplified form a selection of concepts that are furtherdescribed in the detailed description. It is not meant to identify keyor essential features of the claimed subject matter, the scope of whichis defined uniquely by the claims that follow the detailed description.Furthermore, the claimed subject matter is not limited toimplementations that solve any disadvantages noted above or in any partof this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood from reading thefollowing description of non-limiting embodiments, with reference to theattached drawings, wherein below:

FIG. 1 shows an example of an acoustic stack of an ultrasoundtransducer.

FIG. 2 shows an example of a homogeneous multi-element transducer array.

FIG. 3 shows a first graph of an apodization function along an elevationdirection provided by the multi-element transducer array of FIG. 2 .

FIG. 4 shows a first example of a piezoelectric element formed of twosub-elements.

FIG. 5 shows a second example of a piezoelectric element formed of twosub-elements.

FIG. 6 shows a third example of a piezoelectric element formed of twosub-elements.

FIG. 7 shows a first example of a multi-element transducer array withvarying spatial frequency distribution.

FIG. 8 shows a second example of a multi-element transducer array withvarying spatial frequency distribution.

FIG. 9 shows a third example of a multi-element transducer array withvarying spatial frequency distribution.

FIG. 10 shows a first example of an acoustic stack block.

FIG. 11 shows a first comb structure formed from the acoustic stackblock of FIG. 10 .

FIG. 12 shows a second example of an acoustic stack block.

FIG. 13 shows a second comb structure formed from the acoustic stack ofFIG. 12 .

FIG. 14 shows a third example of an acoustic stack block, formed bycoupling the first example of FIG. 10 with the second example of FIG. 12, from a view along the elevation direction.

FIG. 15 shows the third example of the acoustic stack block from a viewalong the azimuth direction.

FIG. 16 shows a coupling of the third example of the acoustic stackblock with a base package from a view along the elevation direction.

FIG. 17 shows the coupling of third example of the acoustic stack blockwith the base package from a view along the azimuth direction

FIG. 18 shows a first example of a base package from a perspective view.

FIG. 19 shows a fourth example of an acoustic stack block, formed fromthe coupling of the third example of the acoustic stack block with thebase package, viewed along the elevation direction.

FIG. 20 shows the fourth example of an acoustic stack block viewed alongthe azimuth direction.

FIG. 21 shows the fourth example of the acoustic stack block of FIG. 19with a portion of a back side of the acoustic stack block ground away,viewed along the elevation direction.

FIG. 22 shows the fourth example of the acoustic stack block of FIG. 20with the portion of the back side of the acoustic stack block groundaway, viewed along the azimuth direction.

FIG. 23 shows the fourth example of the acoustic stack block with aconductive layer coupled to the ground back side, viewed along theelevation direction.

FIG. 24 shows the fourth example of the acoustic stack block with theconductive layer coupled to the ground back side, viewed along theazimuth direction.

FIG. 25 shows a dicing of the fourth example of the acoustic stackblock, viewed along the elevation direction.

FIG. 26 shows the dicing of the fourth example of the acoustic stackblock, viewed along the azimuth direction.

FIG. 27 shows a coupling of a matching layer block to a front side and abacking layer block to a back side of the fourth example of the acousticstack block, viewed along the elevation direction.

FIG. 28 shows the coupling of the matching layer block to the front sideand the backing layer block to the back side of the fourth example ofthe acoustic stack block, viewed along the azimuth direction.

FIG. 29 shows singulation of the fourth example of the acoustic stackblock, viewed along the elevation direction.

FIG. 30 shows singulation of the fourth example of the acoustic stackblock, viewed along the azimuth direction.

FIG. 31 shows a fifth example of a multi-element acoustic stack.

FIG. 32 shows a sixth example of a multi-element acoustic stack.

FIG. 33 shows a seventh example of a multi-element acoustic stack.

FIG. 34 shows an eighth example of a multi-element acoustic stack.

FIG. 35 shows a variation in dicing of the fourth example of theacoustic stack block of FIGS. 29 and 30 .

FIG. 36 shows a combining of two multi-element comb structures to forman acoustic stack with four sub-elements.

FIG. 37 shows an example of a routine for fabricating a multi-frequencyacoustic stack.

FIG. 38 shows an example of a method for forming multi-frequencyelements for the acoustic stack that may be executed as part of theroutine of FIG. 37 .

FIG. 39 shows a second graph of an apodization function along anelevation direction provided by a multi-element transducer array withnon-homogeneous spatial frequency distribution.

FIG. 40 shows a second example of a base package from a perspectiveview.

FIG. 41 shows a third example of a base package from a perspective view.

FIG. 42 shows an example of an acoustic stack formed from combstructures with different kerf dimensions.

DETAILED DESCRIPTION

The following description relates to various embodiments of an acousticstack for a transducer probe. The acoustic stack may be configured witha broad frequency bandwidth by adapting the acoustic stack with apiezoelectric element formed from more than one sub-element. An exampleof an acoustic stack for a transducer probe is shown in FIG. 1 . Each ofthe more than one sub-element may be a different type of element with adifferent resonance frequency. Relative proportions of the more than onesub-element may be maintained constant along both an azimuth directionand an elevation direction of the transducer probe to form a homogeneousarray. An example of a homogeneous multi-frequency transducer array isdepicted in FIG. 2 and a first graph showing a frequency apodizationfunction provided by the homogeneous multi-element (e.g., more than onesub-element) array is illustrated FIG. 3 . In contrast, a taperedapodization function is shown in FIG. 39 which may be produced by amulti-frequency transducer array with varying percent content ofsub-elements included in each element of the transducer array. Asdescribed above, relative proportions of the sub-elements forming thepiezoelectric element may be varied, as shown in FIGS. 4-6 . In someexamples, a multi-frequency transducer array may not be homogeneousalong at least one of the azimuth and elevation directions, insteadexhibiting a varying spatial frequency distribution. Examples ofdifferent spatially distributed multi-frequency transducer arrays areshown in FIGS. 7-9 . A multi-element transducer array may be fabricatedvia a wafer scale approach to enable scalable, low cost manufacturing.Various processes included in the wafer scale approach are depicted inFIGS. 10-36 and 40-42 . An example of a first routine for fabricating amulti-frequency acoustic stack for a transducer probe by the wafer scaleapproach is shown in FIG. 37 . An example of a second routine forforming multi-frequency elements for the acoustic stack is depicted inFIG. 38 and may be included in the first routine of FIG. 37 .

FIGS. 1-2, 4-36, and 40-42 show example configurations with relativepositioning of the various components. If shown directly contacting eachother, or directly coupled, then such elements may be referred to asdirectly contacting or directly coupled, respectively, at least in oneexample. Similarly, elements shown contiguous or adjacent to one anothermay be contiguous or adjacent to each other, respectively, at least inone example. As an example, components laying in face-sharing contactwith each other may be referred to as in face-sharing contact. Asanother example, elements positioned apart from each other with only aspace there-between and no other components may be referred to as such,in at least one example. As yet another example, elements shownabove/below one another, at opposite sides to one another, or to theleft/right of one another may be referred to as such, relative to oneanother. Further, as shown in the figures, a topmost element or point ofelement may be referred to as a “top” of the component and a bottommostelement or point of the element may be referred to as a “bottom” of thecomponent, in at least one example. As used herein, top/bottom,upper/lower, above/below, may be relative to a vertical axis of thefigures and used to describe positioning of elements of the figuresrelative to one another. As such, elements shown above other elementsare positioned vertically above the other elements, in one example. Asyet another example, shapes of the elements depicted within the figuresmay be referred to as having those shapes (e.g., such as being circular,straight, planar, curved, rounded, chamfered, angled, or the like).Further, elements shown intersecting one another may be referred to asintersecting elements or intersecting one another, in at least oneexample. Further still, an element shown within another element or shownoutside of another element may be referred as such, in one example.

Piezoelectric elements may be implemented in transducer probes for awide range of medical applications, including imaging, non-destructivetesting, diagnosis, measuring blood flow, etc. The piezoelectricelements may be formed of a class of crystalline materials that becomeelectrically polarized when subjected to a mechanical strain. Whenstressed, the piezoelectric elements output a voltage that isproportional to the applied stress.

A piezoelectric transducer probe, e.g., a device utilizing apiezoelectric effect to convert energy from one form to another, mayoffer high sensitivity, high frequency response and high transientresponse. In some examples, such as in ultrasound transducer probes, aconverse piezoelectric effect may be leveraged where electricity isapplied to the piezoelectric elements, causing deformation of thematerial and generation of ultrasonic waves. As such, an external,mechanical force is not demanded and the piezoelectric transducer probemay be packaged as a compact, easily transportable device.

Although the piezoelectric transducer probe is a highly sensitiveinstrument, an operational frequency bandwidth of the probe may benarrow. For example, the piezoelectric material may be associated with alow frequency, e.g., between 0.5-2.25 MHz, or a high frequency, e.g.,between 15.0-25.0 MHz, but not both. Similarly, the transducer probe maybe adapted for transmitting or receiving but may not be equipped forhigh performance in both applications due to a focused frequency rangeof the particular type of piezoelectric material. Broadband transducerprobes may provide wider operational frequency ranges but adapting theprobes with electrical impedance matching may be challenging and costprohibitive.

In one example, the issues described above may be addressed by apiezoelectric transducer probe adapted with a multi-frequency transducerarray. The multi-frequency transducer array may include elements in eachtransducer that are formed from more than one sub-element, eachsub-element having a different resonance frequency. In other words, eachelement may be a hybrid element with an overall resonance frequencymodified by the resonance frequencies of the sub-elements. Thus,configuring the transducer array with hybrid elements of varyingcompositions may enable the transducer array operate across a range offrequencies while maintaining a sensitivity and resolution of amulti-frequency transducer probe in which the transducer array isimplemented. Furthermore, the transducers may be fabricated via a waferlevel approach that provides ground recovery, frequency apodization, andfrequency agility in both the azimuth and elevation directions. Aspatial frequency distribution may thereby be controlled and thetransducers may be manufactured through a cost-effective, scalablemanner.

Multi-frequency piezoelectric transducers, as described herein, may beused in a variety of medical devices. For example, as shown in FIG. 1 ,a piezoelectric transducer may be included in an ultrasound probe, usedto create an image based on ultrasonic signals. It will be appreciatedthat the ultrasound probe is a non-limiting example of a medical deviceutilizing the piezoelectric transducer and incorporation of thepiezoelectric transducer in other medical devices have been envisioned.For example, the piezoelectric transducer may be used to convert energyin non-destructive testers, Jetter systems, high voltage power sources,etc. The following description of FIG. 1 is an exemplary overview of howthe piezoelectric transducer may be implemented in the ultrasoundtransducer probe.

An ultrasound probe includes one or more active components forgenerating an ultrasonic signal. An example of an active component, orpiezoelectric element 102 of an ultrasound probe is shown in a schematicdiagram of an acoustic stack 100 in FIG. 1 , with a central axis 104. Aset of reference axes are provided, indicating an azimuth direction 101,an elevation direction 103, and a transverse direction 105 perpendicularto both the azimuth and elevation directions. In other examples, the setof reference axes may represent a z-axis 101, an x-axis 103, and ay-axis 105. The piezoelectric element 102 is shown in FIG. 1 with thecentral axis 104 parallel with the azimuth direction 101.

It will be noted that while the acoustic stack 100 is shown configuredfor a linear ultrasound probe and the azimuth direction is described asparallel with the z-axis in FIG. 1 , other examples may include anazimuth direction that is angled relative to the z-axis, depending on ashape of a piezoelectric element array. For example, the ultrasoundprobe may be curvilinear or phased array, thus generating non-linearbeams that are not parallel with the z-axis.

While a single piezoelectric element is shown in FIG. 1 , the ultrasoundprobe may include a plurality of piezoelectric elements arranged in anarray and individually coupled to an electrical energy source by wires.Each electrical circuit formed of one or more piezoelectric elements maybe a transducer. In some examples, the transducer may include an arrayof piezoelectric elements which may arranged in a variety of patterns,or matrices, including one-dimensional (1D) linear, two-dimensional (2D)square, 2D annular, etc. In one example, the transducer may be formedfrom more than one type of piezoelectric element, thereby providing amulti-frequency piezoelectric transducer. A frequency distribution alongeach of the azimuth and elevation directions may adapted to be uniformor non-uniform. Further details of the multi-frequency piezoelectrictransducer are provided below, with reference to FIGS. 2-42 .

Each transducer may be electrically insulated from adjacent transducersbut may all be coupled to common layers positioned above and below thepiezoelectric element, with respect to the azimuth direction. Theplurality of piezoelectric elements and accompanying layers may beenclosed by an outer housing of the ultrasound probe which may be, forexample, a plastic case with a variety of geometries. For example, theouter housing may be a rectangular block, a cylinder, or a shapeconfigured to fit into a user's hand comfortably. As such, componentsshown in FIG. 1 may be adapted to have geometries and dimensionssuitable to fit within the outer housing of the ultrasound probe.

The piezoelectric element 102 may be a block formed of a naturalmaterial such as quartz, or a synthetic material, such as lead zirconatetitanate, that deforms and vibrates when a voltage is applied by, forexample, a transmitter. In some examples, the piezoelectric element 102may be a single crystal with crystallographic axes, such as lithiumniobate and PMN-PT (Pb(Mg_(1/3)Nb_(2/3))O₃—PbTiO₃). The vibration of thepiezoelectric element 102 generates an ultrasonic signal formed ofultrasonic waves that are transmitted out of the ultrasound probe in adirection indicated by arrows 107, e.g., along the azimuth direction101. The piezoelectric element 102 may also receive ultrasonic waves,such as ultrasonic waves reflected from a target object, and convert theultrasonic waves to a voltage. The voltage may be transmitted to areceiver of the ultrasound imaging system and processed into an image.

Electrodes 114 may be in direct contact with the piezoelectric element102 to transmit the voltage via wires 115, the voltage converted fromultrasonic waves. The wires 115 may be connected to a circuit board (notshown) to which a plurality of wires from electrodes of the plurality ofpiezoelectric elements may be fixed. The circuit board may be coupled toa coaxial cable providing electronic communication between theultrasound probe and the receiver.

An acoustic matching layer 120 may be arranged above the piezoelectricelement 102, with respect to the azimuth direction 101, orientedperpendicular to the central axis 104. The acoustic matching layer 120may be a material positioned between the piezoelectric element 102 and atarget object to be imaged. By arranging the acoustic matching layer 120in between, the ultrasonic waves may first pass through the acousticmatching layer 120, and emerge from the acoustic matching layer 220 inphase, thereby reducing a likelihood of reflection at the target object.The acoustic matching layer 220 may shorten a pulse length of theultrasonic signal, thereby increasing an axial resolution of the signal.

A backing 126 may be arranged below the piezoelectric element 102, withrespect to the z-axis. In some examples, the backing 126 may be a blockof material that extends along the elevation direction 103 so that eachof the plurality of piezoelectric elements in the ultrasound probe aredirectly above the backing 126, with respect to the azimuth direction101. The backing 126 may be configured to absorb ultrasonic wavesdirected from the piezoelectric element 102 in a direction opposite ofthe direction indicated by arrows 107 and attenuate stray ultrasonicwaves deflected by the outer housing of the ultrasound probe. Abandwidth of the ultrasonic signal, as well as the axial resolution, maybe increased by the backing 126.

A piezoelectric transducer (PZT) probe may provide high penetration intoa target as well as high frequency and transient responses, enablinghigh resolution data to be obtained. However, a type of piezoelectricelement included in the probe may operate within a frequency bandwidththat constrains use of the probe to a particular application. Forexample, probe with a low central frequency piezoelectric element may beused to produce ultrasound images of deep tissues or organs but may notprovide sufficient flaw resolution or thickness measurementcapabilities. Thus, use of piezoelectric transducer probes for a varietyof applications may demand access to multiple probes with differentpiezoelectric elements.

In contrast, a capacitive micromachined ultrasonic transducer (CMUT)probe, when used in ultrasonic applications, may offer broader bandwidthas well as more efficient fabrication, due to construction of the CMUTson silicon via micromachining techniques. The broader CMUT bandwidthenables the CMUT probes to achieve greater axial resolution than the PZTprobe. However, a sensitivity and penetration of the CMUT probe may beless than the PZT probe. Furthermore, CMUTs may be more prone toacoustic crosstalk than PZTs.

In one example, high penetration and broad bandwidth may be provided ina PZT probe by adapting the PZT probe with transducers equipped withpiezoelectric elements formed from more than one type of sub-element,each sub-element being a different type of piezoelectric material. Bycombining piezoelectric sub-elements with different resonancefrequencies into one transducer, an array of multi-frequencypiezoelectric elements may be provided. Each of the multi-frequencyelements may have a distinct frequency, depending on relativeproportions of the sub-elements, allowing the multi-frequency elementsto transmit and receive signals over a wider range of frequencies incomparison to a single element transducer probe.

For example, as shown in FIG. 2 , an example of a first matrix 200 withmulti-frequency elements may be a homogeneous bi-dimensional array, suchas a butterfly-type matrix array, with homogeneous multi-frequencyelements 202. The first matrix 200 may represent an arrangement of theelements 202 within a transducer of an acoustic stack, such as theacoustic stack 100 of FIG. 1 . At least one of the acoustic stack may beincorporated within a PZT probe. The first matrix 200 is shown orientedalong the azimuth direction 101 and the elevation direction 103.

Each of the elements 202 includes a first sub-element 204 and a secondsub-element 206. As an example, the first sub-element 204 may be ahigher frequency element and the second sub-element 206 may be a lowerfrequency element, where the first and second sub-elements 204, 206 maybe coupled via a fabrication technique discussed further below, withreference to FIGS. 10-38 . The elements 202 may be spaced apart from oneanother and thereby electrically insulated from adjacent elements 202.Each of the elements 202 may be coupled to an electrical circuit 208 toenable application of a voltage to induce deformation of each of theelements 202. Furthermore, each of the elements 202 may transmit anindividual signal, as induced by deformation. It will be noted that eachof the elements 202 are coupled to the electrical circuit 208 but onlythe bottom row of elements 202 are shown directly coupled to theelectrical circuit 208 in FIG. 2 for brevity.

The first matrix 200 may be coupled to other layers of the acousticstack, e.g., an acoustic lens, a backing, etc., as shown in FIG. 1 . Asa result of forming the elements from the first sub-element 204 and thesecond sub-element 206, the elements may transmit and/or receive acrossa wide range of frequencies. For example, the first sub-element 204 mayhave a central (e.g., resonance) frequency of 2.0 MHz and the secondsub-element 206 may have a central frequency of 15 MHz. By combining thefirst sub-element 204 with the second sub-element 206 with equalrelative proportions, the elements 202 may transmit and/or receiveultrasonic signals across a wider range of frequencies than either thefirst sub-element 204 or the second sub-element 206 alone.

For example, the each of the elements 202 may have a frequency range of1.5 to 15 MHz. The array of the elements 202 in the first matrix 200 mayprovide symmetric and linear apodization functions along the elevationdirection 103, as shown in FIG. 3 . Apodization provided by both a highfrequency sub-element (e.g., the first sub-element 204 of FIG. 2 ), asindicated by plot 302, and a low frequency sub-element (e.g., the secondsub-element 206 of FIG. 2 ), as indicated by plot 304, are illustratedin graph 300. The apodization functions are plotted relative to percentcontent of the high frequency and low frequency sub-elements in eachelement of a transducer array along the y-axis and to the elevationdirection 103 along the x-axis. The equal proportions of the highfrequency sub-element and low frequency sub-element distributed alongthe elevation direction results in a uniform apodization function.

As described above, a symmetric and non-tapering apodization functionmay be provided by the high frequency and low frequency sub-elements204, 206 of the elements 202 of FIG. 2 with equal proportions of thesub-elements in a homogeneous matrix array, e.g., each multi-frequencyelement of the array is configured similarly. However, in order to bringa sampled signal down to zero, or near zero, at edges of a sample regionto suppress leakage sidelobes, a tapering of the apodization functionmay be desired. For example, a tapered apodization function(non-discretized) provided by an element formed of a high frequencysub-element and a low frequency sub-element is shown in a second graph3900 in FIG. 39 .

Plot 3902 represents an apodization function of the high frequencysub-element in each element of a transducer array relative to percentcontent (y-axis) along the elevation direction 103 and plot 3904represents an apodization function of the low frequency sub-element ineach element of the transducer array. Plot 3902 and plot 3904 areinversely correlated so that a maximum of the first plot 3902, relativeto the y-axis, at a central region of the transducer array along theelevation direction 103 corresponds to a minimum of plot 3904. To eitherside of the maximum, plot 3902 decreases along the y-axis and the plot3904 increases proportionally. While the maximum percent content of eachof the high frequency and low frequency sub-elements is shown at 80% andthe minimum percent content is shown at 20%, other examples may includeany other values of the maximum and minimum percent content, such as100% and 0%, respectively.

A sum of the high frequency and low frequency apodization functions mayprovide side lobe reduction. For example, it may be desirable to have ahigher proportion of high frequency elements at a central region of thetransducer array and higher proportion of the low frequency elementsalong the sides of the transducer array to achieve maximum suppressionof leakage side lobes. The tapered apodization function shown in graph3900 may be generated by configuring the elements of the transducerarray with unequal relative proportions of each of the high and lowfrequency sub-elements. For example, a central region of the transducermay include elements with higher percent content of the high frequencysub-element than the outer edges. Suppression of side lobes is therebyenhanced.

Examples of elements with unequal sub-element proportions are shown inFIGS. 4-6 . A first example of a multi-frequency element 400 withunequal distribution is shown in FIG. 4 . The multi-frequency element400 is formed of a first, high frequency sub-element 402 and a second,low frequency sub-element 404, similar to the first and secondsub-elements 204, 206 of the elements 202 of FIG. 2 .

A first width 406 of the first sub-element 402 may be greater than asecond width 408 of the second sub-element 404. For example, the firstwidth 406 may be four times greater than the second width 408, resultingin 80% of the multi-frequency element 400 formed of the first, highfrequency sub-element 402 and 20% formed of the second, low frequencysub-element 404. The relative widths of the sub-elements may be reversedin other examples. A second example of a multi-frequency element 500 isshown in FIG. 5 and includes a first, high frequency sub-element 502 anda second, low frequency sub-element 504. In the second example, a firstwidth 506 of the first sub-element 502 may be half of a second width 508of the second sub-element 504.

In a third example of a multi-frequency element 600 with unequaldistribution, as illustrated in FIG. 6 , the multi-frequency element 600is similarly formed of a first, high frequency sub-element 602 and asecond, low frequency sub-element 604. A first width 606 of the firstsub-element 602 is also less than a second width 608 of the secondsub-element 604. The first width 606 may be, for example, a quarter ofthe second width 608.

The examples of multi-frequency elements with unequal distribution ofsub-elements shown in FIGS. 4-6 are non-limiting examples ofmulti-frequency elements. Other examples, may include multi-frequencyelements with any variation in relative proportions between the firstsub-element and the second sub-element of the elements. Furthermore,other examples of the multi-frequency elements may include more than twosub-elements. For example, the multi-frequency elements may be formed ofthree or four sub-elements, with a variety of proportions of each of thesub-elements.

An array of non-homogeneous multi-frequency elements may be configuredto provide the frequency apodization function shown in FIG. 39 . Thearray may be non-homogeneous with respect to a percent content ofdifferent sub-elements forming each multi-frequency element included inthe array. In other words, a number of sub-elements, each of thesub-elements having a different resonance frequency, as well as relativeproportions of the sub-elements in each element may not be uniformacross the array, enabling a spatial distribution of elements withdifferent frequency ranges. In one example, as shown in FIG. 7 , asecond matrix 700, configured to provide an apodization function similarto that shown in graph 3900 of FIG. 39 , may be an example of aone-dimensional (1D) linear array formed of a plurality of elements 702.A first row 701, a second row 703, and third row 705 of the plurality ofelements 702 are depicted. Dotted lines between the second row 703 andthe third row 705 represent a presence of optional additional rowsarranged between the second row 703 and the third row 705, omitted fromFIG. 7 for brevity. In other words, the second matrix 700 may have atleast one row and may include any number of additional rows. Each of theplurality of elements 702 may be coupled to an electrical circuit andtransmit an individual signal based on a composition of each element.

Each of the plurality of elements 702 includes a first, high-frequencysub-element 704 and/or a second, low-frequency sub-element 706. Some ofthe plurality of elements 702 include both the first sub-element 704 andthe second sub-element 706 with varying widths of the sub-elementsrelative to one another (where the width is defined along the elevationdirection 103). As well, some of the plurality of elements 702 includeonly the first sub-element 704 or only the second sub-element 706.

For example, a central region 720 of the second matrix 700 includes aportion of the plurality of elements 702 formed of only the firstsub-element 704 while edge regions 722, distal to a central axis 708 ofthe second matrix 700, are formed of only the second sub-element 706.Regions 724 of the second matrix 700 between the central region 720 andthe edge regions 722 are formed of both the first sub-element 704 andthe second sub-elements 706 in varying ratios. As a result of a spatialdistribution of the first and second sub-elements 704, 706 along theelevation direction 103, a resonance frequency of each of the pluralityof elements 702 may vary along the elevation direction 103.

For example, in the central region 720 of the second matrix 700, theportion of the plurality of elements 702 that include only the firstsub-element 704 may each transmit (and receive) signals at a resonancefrequency associated with the first sub-element 704. At the edge regions722, the portion of the plurality of elements 702 formed from only thesecond sub-element 706 may each transmit (and receive) signals at aresonance frequency associated with the second sub-element 706. In theregions 724 between the central region 720 and the edge regions 722, theplurality of elements 702 are hybrids, e.g., combinations of the firstand second sub-elements 704, 706, and may therefore have a range ofresonance frequency values in between those of the first and secondsub-elements 704, 706.

As an example, a first element 707 of the plurality of elements 702 maybe composed of 50% of the first sub-element 704 and 50% of the secondsub-element 706. The resonance frequency of the first element 707 may bea value mid-way between that of the first sub-element 704 and the secondsub-element 706. A second element 709 of the plurality of elements 702,positioned between the first element 707 and the central region 720 ofthe second matrix 700, may have a higher percent composition of thefirst sub-element 704 compared to the first element 707. The secondelement 709 may therefore have a resonance frequency that is higher thanthe first element 707 but lower than the resonance frequency of thefirst sub-element 704. A third element 711 of the plurality of elements702, positioned between the first element 707 and the left-hand edgeregion 722 may have a higher percent composition of the secondsub-element 706 than the first element 707. The third element 711 maythere have a resonance frequency that is lower than the first element707 but higher than the second sub-element 706.

By incrementally changing the composition of the plurality of elements702 along the elevation direction 103, the plurality of elements 702 mayhave a continuum of resonance frequencies ranging between the resonancefrequency of the first sub-element 704 and the resonance frequency ofthe second sub-element 706. In other examples, the composition of theplurality of elements 702 may be similarly varied along the azimuthdirection 101 instead of the elevation direction 103 or in addition tothe elevation direction 103. Thus the second matrix 700 may transmit andreceive signals through a wider range of frequencies than a transducerarray with a uniform element composition. In the example shown in FIG. 7, highest frequencies may be transmitted and received at the centralregion 720 of the second matrix while lowest frequencies may betransmitted and received at the edge regions 722.

The second matrix 700 may be symmetric about the central axis 708 of thesecond matrix 700, the central axis parallel with the azimuth direction101. The symmetry of the second matrix 700, regardless of variations indistribution of the sub-elements amongst the plurality of elements 702,allows the second matrix to provide the apodization function as shown inFIG. 39 .

An example of a third matrix 800 is illustrated in FIG. 8 , which may bean example of a 1.5-dimensional (1.5D) matrix array. The third matrix800 includes a plurality of elements 802 arranged in a first row 804, asecond row 806, and a third row 808. Similarly, dotted lines between thefirst row 804 and the second row 806 indicate a presence of additionalrows of the third matrix 800, omitted for brevity. The third matrix 800has a central axis 810 parallel with the azimuth direction 101.

At least a portion of the plurality of elements 802 of the third matrix800 may be multi-frequency elements 816 formed of a first,high-frequency sub-element 812 and a second, low-frequency sub-element814. For example, the multi-frequency elements 816 of the plurality ofelements 802 may include two of the first sub-element 812 alternatingwith two of the second sub-element 814 along the elevation direction103. A central region 820 of the third matrix 800 may be formed only ofthe first sub-element 812 while edge regions 822 of the third matrix 800may be formed of only the second sub-element 814. Furthermore, athickness of each of the plurality of elements 802, defined along theelevation direction 103, may vary across each row of the third matrix800.

As a result of a spatial distribution of the first and secondsub-elements 812, 814 along the elevation direction 103, a resonancefrequency of the plurality of elements 802 may vary across the elevationdirection 103. For example, similar to the second matrix 700 of FIG. 7 ,a portion of the plurality of elements 802 in the central region 820 maytransmit and receive signals at a higher resonance frequency equal to aresonance frequency of the first sub-element 812, while a portion of theplurality of elements 802 at the edge regions may transmit and receivesignals at a lower resonance frequency equal to a resonance frequency ofthe second sub-element 814. A portion of the plurality of elements 802between the central region 820 and the edge regions 822 may haveintermediate resonance frequencies between those of the firstsub-element 812 and the second sub-element 814. Thus a range offrequencies encompassed by the third matrix 800 may be broadened incomparison to use of a single frequency element in the array of thethird matrix 800.

The third matrix 800 may be symmetric across the central axis 810, alongthe elevation direction 103. Similar to the second matrix 700 of FIG. 7, the symmetry of the third matrix 800 enables the third matrix 800 toprovide the apodization function as shown in FIG. 39 . The 1.5D array(as well as a 1.75D array) may provide an optimized beam pattern as anactive aperture of a transducer probe changes. As such, the array isable to optimize a near field with a narrow aperture as well as a farfield with a larger aperture. By manufacturing the 1.5D or 1.75D arrayvia a process shown in FIGS. 10-38 , the process allows mixing ofelements with different central frequencies and frequency ranges withina single array. The manufacturing process provides increased flexibilityin array configuration with incurring extensive additional costs. Anexample of a fourth matrix 900, which may be an example of a 1.25 lineararray, is shown in FIG. 9 . The fourth matrix 900 also has a pluralityof elements 902, arranged in rows along the elevation direction 103, anda central axis 904, parallel with the azimuth direction 101. Theplurality of elements 902 may each be formed of a single type of elementand are not hybrid elements.

The fourth matrix 900 includes a first, high-frequency sub-element 906and a second, low-frequency sub-element 908. Each of the plurality ofelements 902 may be formed of either the first sub-element 906 or thesecond sub-element 908 and may vary in width along the elevationdirection 103. A symmetry of the fourth matrix 900 across the centralaxis 904 along the elevation direction 103 also allows the fourth matrix900 to provide apodization along the elevation direction 103.Incorporation of more than one type of element allows the fourth matrix900 to operate across a wider range of frequencies. However, thefrequency distribution may be less continuous and more discretized thanthe matrices of FIGS. 7 and 8 without incorporation of hybrid elements,e.g., elements formed of more than one sub-element.

It will be noted that each element of the plurality of elements of FIGS.7-9 may be coupled to an electrical circuit, as shown in FIGS. 1 and 2 .Additionally, other examples of a multi-frequency transducer array mayalso include control of frequency apodization and agility along theazimuth direction 101. For example, a distribution of multi-frequencyelements may be varied along the azimuth direction 101 in a similarmanner as shown along the elevation direction 103. Varying a spatialfrequency distribution along the azimuth direction may be implemented inan array as an alternative or in addition to frequency variation alongthe elevation direction. By configuring the array with multi-frequencyelements along both the azimuth and elevation directions, more complexapodization is enabled relative to frequency apodization in a transducerarray with uniform elements. By providing a method to vary the elementsalong both directions, a configuration of the array is more flexible andmay be implemented more easily as a matrix. Furthermore, the frequencyagility, e.g., an ability of a transducer to quickly shift a transmittedfrequency over a pre-selected range to mitigate jamming, mutualinterference or account for atmospheric effects, may be enabled alongboth the azimuth and elevation directions.

Incorporation of multi-frequency elements into a transducer array mayenable enhanced sensitivity for both transmission and reception ofsignals while increasing a frequency bandwidth of the transducer array.Signal transmission at a specific frequency, based on an application ofa transducer probe, may be selected, resulting in energization ofmulti-frequency elements in the transducer array with a correspondingresonance frequency. By configuring the transducer array with elementswith a broad range of frequencies, different operation of the transducerprobe is enabled. The transducer probe may thereby be used for a varietyof applications that would otherwise demand use of multiple singleelement transducer probes with different resonance frequencies.

In some examples, post-processing of signals received by the transducerarray may be similar to conventional post-processing, utilizing alreadyexisting post-processing algorithms to convert the signals into, forexample, images. Bandpass filtering of the signals may be modified basedon the frequency of the signal.

Fabrication of an array of multi-frequency transducer elements may beachieved via a cost-effective process leveraging a wafer level approach.The wafer level approach allows multiple transducer arrays to begenerated simultaneously, thereby increasing efficiency and throughput.A fabrication process for a multi-frequency transducer array is nowdescribed with reference to FIGS. 10-38 and 40-42 . The wafer levelapproach may begin with a block of a first acoustic stack 1000, as shownin FIG. 10 . The first acoustic stack 1000 is viewed along the elevationdirection in FIG. 10 and includes a matching layer 1002, similar to theacoustic matching layer 120 of FIG. 1 , which may be an electricallyconductive layer such as graphite or a metal. The matching layer 1002may be formed of more than one layer stacked along a vertical axis ofthe first acoustic stack 1000, e.g., along the transverse direction 105,configured to be electrically conductive along the vertical axis.

The matching layer 1002 is arranged above, relative to the transversedirection 105, a first piezoelectric layer 1004. An acoustic impedancedifference between an ultrasound transducer probe and a target mediummay be buffered by the matching layer 1002. The first piezoelectriclayer 1004 is formed of a piezoelectric material configured to transmitand/or receive ultrasound signals and used to form transducer elementsof an ultrasound transducer probe, as described above.

A dematching layer 1006 may be positioned below the first piezoelectriclayer 1004. The dematching layer may be a high impedance layer that maydecrease insertion losses and enhance a frequency bandwidth of atransducer probe. In some examples, the dematching layer may beoptionally omitted. A backing layer 1008, similarly to the backing 126of FIG. 1 , may be arranged below the dematching layer 1006. The backinglayer 1008 may be formed of an electrically conductive material, such asa composite, for example, and may dampen a ringing effect which mayoccur when the piezoelectric material switches from a transmission modeto a receiving mode. The first piezoelectric layer 1004 may be bonded tothe matching layer 1002 and to the dematching layer 1006 (or to thebacking layer 1008 when the dematching layer 1006 is not present) withan adhesive such as epoxy.

The first piezoelectric layer 1004 of the first acoustic stack 1000 mayhave a first height 1010, defined along the transverse direction 105.The first height 1010 may visually differentiate a piezoelectric elementwith a higher resonance frequency from a resonance frequencypiezoelectric element of a second piezoelectric layer 1204 with a largersecond height 1210, shown in FIGS. 12 and 13 and described furtherbelow. A first comb structure 1100 may be produced from the firstacoustic stack 1000 of FIG. 10 by dicing kerfs 1102 evenly spaced alongthe transverse direction 105 on the first acoustic stack 1000. The kerfs1102 extend downwards, along the azimuth direction 101, from thematching layer 1002 into the backing layer 1008 but not entirely throughthe backing layer 1008. Dicing of the kerfs 1102 forms first fins 1104,each of the first fins 1104 spaced apart from adjacent first fins 1104by one of the kerfs 1102. The first fins 1104 extend upwards along theazimuth direction 101 from the backing layer 1008 and may extend alongthe elevation direction 103 across an entire depth of the first combstructure 1100.

It will be noted that dicing refers to cutting of kerfs into a wafer toform cavities or slots in the wafer that do not extend entirely througha height of the wafer. Thus dicing may electrically isolate portions ofthe wafer, e.g., renders a section electrically discontinuous from anadjacent section along a plane perpendicular to the height, but does notdivide the wafer into individual, separate sections. In contrast,singulation facilitates singularizing of the wafer into individualtransducer arrays that are physically separated, as described below.Herein, dicing and singulation are conducted only along the height ofthe wafer, e.g., along the transverse direction so that portions of thewafer are electrically isolated and/or physical separated only along theplane formed by the azimuth and the elevation directions.

The first comb structure 1100 may be diced into a geometry to complementor match a second comb structure (e.g., a second comb structure 1300shown in FIG. 13 . The second comb structure 1300 may be formed from asecond acoustic stack 1200, depicted in FIG. 12 . The second acousticstack 1200 may have similar layers to the first acoustic stack 1000,including a matching layer 1202, formed of a same or different material(or stack of electrically conductive layers) as the matching layer 1002of the first acoustic stack 1000, a second piezoelectric layer 1204,formed of a different material as the first piezoelectric layer 1004 ofFIG. 10 , an optional dematching layer 1206, similar to the dematchinglayer 1006 of the first acoustic stack 1000, and a backing layer 1208,formed of a same or different material as the backing layer 1008 of thefirst acoustic stack 1000.

As described above, the second height 1210, defined along the azimuthdirection 101, of the second piezoelectric layer 1204 may be greaterthan the height 1010 of the first piezoelectric layer 1004 of the firstacoustic stack 1000. Piezoelectric elements formed from the secondpiezoelectric layer 1204 may have a lower resonance frequency than thepiezoelectric elements formed from the first piezoelectric layer 1004.The diced piezoelectric elements corresponding to the firstpiezoelectric layer 1004, e.g., in the first comb structure 1100, arehereafter referred to as high frequency sub-elements 1004 and the dicedpiezoelectric elements corresponding to the second piezoelectric layer1204, e.g., in the second comb structure 1300, are hereafter referred toas low frequency sub-elements 1204.

A height, also defined along the azimuth direction 101, of the matchinglayer 1202 of the second acoustic stack 1200 may be greater than aheight of the matching layer 1002 of the first acoustic stack 1000 whilea height of the backing layer 1208 of the second acoustic stack 1200 maybe less than a height of the backing layer 1008 of the first acousticstack 1000. The difference in heights between the matching layers andthe backing layers may allow the layers of each of the first and secondcomb structures 1100, 1300 to have a desired alignment when the combstructures are combined into a single structure, described furtherbelow.

The second acoustic stack 1200 may be diced in an opposite directionfrom the first acoustic stack 1000, as shown in FIG. 13 . As such, thesecond acoustic stack 1200 is diced so that kerfs 1302 extend from thebacking layer 1208 upwards, along the azimuth direction 101, into thematching layer 1202. The kerfs 1302 do not extend entirely through thematching layer 1202. The kerfs 1302 are evenly spaced apart along thetransverse direction 105, forming second fins 1304 between each of thekerfs 1302. The second fins 1304 may extend across an entire depth ofthe second comb structure 1300 along the elevation direction 103.

A width 1306 of each of the kerfs 1302 of the second comb structure 1300may be equal to a width 1106 (as shown in FIG. 11 ) of each of the firstfins 1104 of the first comb structure 1100. Similarly, a width 1308 ofeach of the second fins of the second comb structure 1300 may be equalto a width 1108 (as shown in FIG. 11 ) of each of the kerfs 1102 of thefirst comb structure 1100. A height 1310, defined along the azimuthdirection 101, of both the kerfs 1302 of the second comb structure 1300and the second fins 1304 may equal a height of both the first fins 1104and the kerfs 1102 of the first comb structure 1100. The complementarygeometries of the first comb structure 1100 and the second combstructure 1300 allow the comb structures to fit together to form a thirdacoustic stack 1402 with interdigitated comb structures, as shown inFIG. 14 from a first view 1400 along the elevation direction 103 and inFIG. 15 from a second view 1500 along the azimuth direction 101.

In the first view 1400 of the third acoustic stack 1402, illustrated inFIG. 14 , a first layer of adhesive 1404 is arranged between the firstcomb structure 1100 and the second comb structure 1300 to enablelamination of the comb structures. The first layer of adhesive 1404 maybe a non-conductive glue, such as epoxy, that electrically insulates thefirst comb structure 1100 from the second comb structure 1300. The firstcomb structure 1100 and the second comb structure 1300 may be nestedinto one another so that there are no gaps between the first combstructure 1100 and the second comb structure 1300.

As shown in the second view 1500 of the third acoustic stack 1402, thefirst fins 1104 and the second fins 1304, each fin forming a digit ofthe interdigitated structure of the third acoustic stack 1402, extendsalong a depth 1502 of the third acoustic stack 1402 along the elevationdirection 103. It will be appreciated that the first view 1400 and thesecond view 1500 of the third acoustic stack 1402 may represent asection of the third acoustic stack rather than the entire acousticstack 1402. While the third acoustic stack 1402 is shown with three ofthe first fins 1104 and three of the second fins 1304 in FIG. 14 , thethird acoustic stack 1402 may have any number of the fins. The thirdacoustic stack 1402 may have a width 1406 and depth 1502 greater or lessthan shown in FIGS. 14 and 15 , respectively.

Additionally, in some examples, the third acoustic stack 1402 may befurther combined with one or more additional comb structures to increasea number of sub-elements with different resonance frequenciesincorporated into an acoustic stack. For example, as shown in FIG. 36 ,a first multi-frequency comb structure 3602 and a second multi-frequencycomb structure 3604 may each be formed from an acoustic stack such asthe third acoustic stack 1402.

At least one first fin 3603 of the first multi-frequency comb structure3602 may include a first sub-element 3606 and a second sub-element 3608.The first multi-frequency comb structure 3602 may be formed by dicing anacoustic stack similarly to the dicing of the first acoustic stack 1000as shown in FIG. 11 , with a first kerf 3610 extending downwards from atop of the first multi-frequency comb structure 3602, along thetransverse direction 105, through a portion of a height 3612 of thefirst multi-frequency comb structure 3602.

The second multi-frequency comb structure 3604 may have at least onesecond fin 3614, the second fin 2614 including a third sub-element 3616and a fourth sub-element 3618. Each of the first, second, third, andfourth sub-elements 3606, 3608, 3616, 3618 may have different resonancefrequencies. The second multi-frequency comb structure 3604 may be dicedsimilarly to the second acoustic stack 1200 as shown in FIG. 13 , with asecond kerf 3620 extending upwards from a bottom of the secondmulti-frequency comb structure 3604, along the transverse direction 105,through a portion of a height 3622 of the second multi-frequency combstructure 3604.

A width 3624 and a height 3626 of the first kerf 3610 may be similar toa width and a height of the second fin 3614. A width 3628 and a height3630 of the second kerf 3620 may be similar to a width and a height ofthe first fin 3603. The second fin 3614 of the second multi-frequencycomb structure 3604 may be inserted into the first kerf 3610 of thefirst multi-frequency comb structure 3602 while the first fin 3603 maybe inserted into the second kerf 3620 of the second multi-frequency combstructure 3604, as indicated by arrows 3632 to form a combined stackwith four sub-elements. The combined stack may be laminated and furtherprocessed as described below.

FIG. 36 shows a non-limiting example of how a multi-frequency acousticstack with four different sub-elements may be formed. In other examples,either of the first multi-frequency comb structure 3602 or the secondmulti-frequency comb structure 3604 may be a single element combstructure. In such instances, a resulting combined acoustic stack mayinclude three sub-elements. Furthermore, widths, defined along theazimuth direction 101, of each of the sub-elements are shown to besimilar, resulting in a combined stack with equal proportions of eachsub-elements. In other examples, however, the widths of the sub-elementsmay be varied so that the percent content of each of the sub-elements isnot equal.

Furthermore, while the third acoustic stack 1402 of FIGS. 14-15 show thefirst and second comb structures 1100, 1300 having complementarygeometries that result in a gap-free combining of the comb structure,e.g., no spaces are present between the comb structures when coupled,the comb structures may be diced to have non-matching geometries. Forexample, as shown in FIG. 42 , an alternate example of an acoustic stack4200 may include a first comb structure 4202 and a second comb structure4204, combined to form an interdigitated structure.

Kerfs of the first comb structure 4202 may not have dimensions thatmatch dimensions of fins of the second comb structure 4204 and kerfs ofthe second comb structure 4204 may not have dimensions matching fins ofthe first comb structure 4202. For example a first kerf 4206 of thefirst comb structure 4202 may have a depth 4208 that is greater than adepth 4210 of a first fin 4212 of the second comb structure 4204. Whenthe first fin 4212 of the second comb structure 4204 is inserted intothe first kerf 4206 of the first comb structure 4204, gaps may bepresent around the first fin 4212, e.g., along the azimuth direction.

A second kerf 4214 of the first comb structure 4202 may also have adepth 4216 that is greater than a depth 4218 of a second fin 4220 of thesecond comb structure 4204. When the second fin 4220 of the second combstructure 4204 is inserted into the second kerf 4214 of the first combstructure 4202, gaps may be present around the second fin 4220, e.g.,along the azimuth direction 101. The gaps around the second fin 4220 maybe greater than the gaps around the first fin 4212 due to eithernon-uniform depths of the kerfs of the first comb structure 4204 and/ornon-uniform depths of the fins of the second comb structure 4204. Thefins of the first comb structure 4202 may be similarly surrounded bygaps due to greater depths of the kerfs of the second comb structure4204 compared to depths of the fins of the first comb structure 4204.

As shown in FIG. 42 , a variety of geometries of the acoustic stack,formed by combining at least two comb structures, may be enabled byadjusting dimensions of the kerfs and fins. The dicing and combining ofthe comb structures introduces a high degree of flexibility in a finalconfiguration of a transducer array. Thus modification of the transducermay be efficiently modified.

Turning now to FIGS. 16-17 , the third acoustic stack 1402 may becombined with a first example of a base package 1602, as shown in afirst view 1600 along the elevation direction 103 in FIG. 16 and in asecond view 1700 along the azimuth direction 101 in FIG. 17 . The basepackage 1602 may be formed from a conductive material such as graphite,porous graphite filled with resin, stainless steel, aluminum etc. Thebase package 1602 may be diced to have first fins 1604, extending alongthe transverse direction 105, and kerfs 1606. The third acoustic stack1402 may also be diced to have first kerfs 1608, also extending alongthe elevation direction 103, that match the first fins 1604 of the basepackage 1602 in a width 1610 and a height 1612. Dicing of the thirdacoustic stack 1402 also forms blocks 1614 with the same height 1612 asthe first fins 1604 of the base package 1602 and with a width 1616 ofeach of the blocks 1614 equal to a width of the kerfs 1606 of the basepackage 1602.

The dicing of the third acoustic stack 1402 and the base package 1602are further shown in FIG. 17 . The third acoustic stack 1402 may havesecond kerfs 1702, extending along the azimuth direction 101, inaddition to the kerfs 1606. The base package 1602 has second fins 1704which may be continuous with the first fins 1604 but extending along aperpendicular direction from the first fins 1604, e.g., along theazimuth direction 101. As such, the first fins 1604 and the second fins1704 may form a structure as shown in a perspective view 1800 in FIG. 18.

The base package 1602 is depicted in the perspective view 1800 to showan overall geometry of the first fins 1604, the second fins 1704 and thekerfs 1606 of the base package 1602. The first fins 1604 and the secondfins 1704 frame each of the kerfs 1606 so that each of the kerfs 1606has a uniform rectangular geometry. In other examples, however, thekerfs 1606 may have a variety of other geometries, such as circular,hexagonal, square, etc. As such, transducers produced by themanufacturing process depicted in FIGS. 10-35 may have a shapecorresponding to a geometry of the kerfs 1606.

The kerfs 1606 form cavities in the base package 1602 and the blocks1614 of the third acoustic stack 1402 (as shown in FIGS. 16 and 17 ) areshaped to match the geometry of the kerfs 1606. In this way, the kerfs1606 of the base package 1602 receive the blocks 1614 of the thirdacoustic stack 1402, as indicated by arrows 1618 in FIGS. 16 and 17 ,the first kerfs 1608 of the third acoustic stack 1402 receive the firstfins 1604 of the base package 1602, and the second kerfs 1702 of thethird acoustic stack 1402 receive the second fins 1704 of the basepackage 1602.

In other examples, a base package may be configured differently than thebase package 1602 of FIG. 18 . For example, as shown in FIG. 40 , asecond example of a base package 4000, may have kerfs 4002 extendinglinearly and continuously along the elevation direction 103. The kerfs4002 are parallel and may extend across an entire depth of the basepackage 4000, the depth defined along the elevation direction 103, oracross at least a portion of the depth. Fins 4004 of the base package4000, spaced apart by the kerfs 4002, may also extend along theelevation direction 103. An acoustic stack may be similarly diced alongthe elevation direction 103 to match the kerfs 4002 and fins 4004 of thebase package 4000.

Alternatively, a base package may be diced entirely along the azimuthdirection 101, as shown in FIG. 41 . FIG. 41 depicts a third example ofa base package 4100 with kerfs 4102 and fins 4104 extending along theazimuth direction 101. The kerfs 4102 and fins 4104 may extend entirelyor partially across a width of the base package 4100, the width definedalong the azimuth direction 101. An acoustic stack may be diced withkerfs and blocks to match a geometry of the base package 4100, e.g.,with kerfs and blocks extending along the azimuth direction 101.

Turning now to a first view 1900 along the elevation direction in FIG.19 and a second view 2000 along the azimuth direction 101 in FIG. 20 ,the third acoustic stack 1402 and the base package 1602 may be laminatedwith a second layer of adhesive 1904, disposed between the thirdacoustic stack 1402 and the base package 1602, to form a fourth acousticstack 1902. The second layer of adhesive 1904 is illustrated as a dottedline to differentiate the second layer of adhesive 1904 from the firstlayer of adhesive 1404. The second layer of adhesive 1904 may also be anon-conductive glue that electrically isolates the third acoustic stack1402 from the base package 1602.

As shown in a first view 2100 of the fourth acoustic stack 1902 alongthe elevation direction 103 in FIG. 21 and in a second view 2200 of thefourth acoustic stack 1902 along the azimuth direction in FIG. 22 , aback side 2102 of the fourth acoustic stack 1902 may be subjected togrinding. Ground recovery in both the elevation direction 103 and theazimuth direction 101 is enabled by grinding the back side 2102 so thata portion of the base package 1602, a portion of the backing layer 1008of the first comb structure 1100, and a portion of the backing layer1208 of the second comb structure 1300 (the backing layers 1008, 1208shown in FIGS. 14-15 ) is removed. The back side 2102 of the fourthacoustic stack may provide a positive terminal connectivity. A height1906 of an overall portion of the fourth acoustic stack 1902 that isground away is shown in FIGS. 19 and 20 .

The back side 2102 of the fourth acoustic stack 1902 is ground untilportions of both the first layer of adhesive 1404 and the second layerof adhesive 1904 that are parallel with the elevation direction 103 (asshown in FIGS. 19 and 20 ) are removed. By removing the portions of theadhesive layers, e.g., bottom portions of the adhesive layers relativeto the azimuth direction 101, ground recovery in both the elevation andazimuth directions is enabled. In other words, electrical continuitybetween the elements (e.g., the high frequency sub-elements 1004 and thelow frequency sub-elements 1204) and electrical contacts or electrodes(not shown in FIGS. 21 and 22 ), arranged in contact with the back side2102 of the fourth acoustic stack 1902, is provided by removing theinsulating adhesive layers.

Ground recovery may further include sputtering a layer of anelectrically conductive material, such as a metal, on the back side 2102of the fourth acoustic stack 1902, as shown in FIG. 23 in a first view2300 along the elevation direction 103 and in FIG. 24 in a second view2400 along the azimuth direction 101. The fourth acoustic stack 1902 isdepicted in FIGS. 23 and 24 flipped upside, relative to the azimuthdirection 101. A sputtered layer 2302 is deposited onto the back side2102 of the fourth acoustic stack 1902, forming a uniform, continuousfilm. A height, measured along the azimuth direction, of the sputteredlayer 2302 is less than the heights of the any of other layers, e.g.,the matching layers, the high and low frequency elements, the dematchinglayers, the backing layers, of the fourth acoustic stack 1902.

In other examples, however, sputtering may be precluded by grinding theback side of the fourth acoustic stack 1902 to a lesser extent, so thata portion of the base package 1602 remains. For example, the back sidemay be ground by an amount indicated by arrow 1907 shown in FIG. 19 .The remaining portion of the base package may be common to each of thesub-elements and may provide an electrically conductive layer along thebackside of the fourth acoustic stack 1902.

The fourth acoustic stack 1902 may be diced after deposition of thesputtered layer 2302, as shown in FIG. 25 in a first view 2500 of thefourth acoustic stack 1902 along the elevation direction 103 and in FIG.26 in a second view 2600 along the azimuth direction 101. A plurality ofkerfs 2503 are formed in the fourth acoustic stack 1902, extending fromthe back side 2302 towards a front side 2502 of the fourth acousticstack 1902 but not entirely through the fourth acoustic stack 1902,along the transverse direction 105.

The plurality of kerfs 2503 may separate the fourth acoustic stack 1902into a plurality of elements 2501. The plurality of elements 2501 mayinclude multi-frequency elements 2504 and single frequency elements2506, as shown in FIG. 25 . The multi-frequency elements 2504 eachinclude one of the high frequency sub-elements 1004 and one of the lowfrequency sub-elements 1204. The single frequency elements 2506 includeeither one of the high frequency sub-elements 1004 or one of the lowfrequency sub-elements 1204 but not both.

In other examples, each element of an acoustic stack, such as the fourthacoustic stack 1902, may be formed from a single element but theacoustic stack may include various different types of single elements.For example, a first and second comb structure may be combined to form asimilar acoustic stack as the third acoustic stack 1402 of FIGS. 14-15 .The acoustic stack may be processed as described above with reference toFIGS. 16-24 , and kerfs may be diced into the acoustic stack, as shownin FIGS. 25 and 26 . However, the kerfs may be positioned between eachfin of each comb structure, thus separating the elements into singleelement digits of the diced acoustic stack. In other words kerfs mayalso separate the high frequency sub-elements 1004 from the lowfrequency sub-elements 1204. In this way, the acoustic stack may be amulti-frequency acoustic stack with single elements, rather thanelements formed from more than one sub-element, where each transducermay have more than one type of element, each element coupled to anelectrical circuit.

Returning to FIG. 25 , a resonance frequency of the multi-frequencyelements 2504 may be determined by a percent content of each of the highand low frequency sub-elements 1004, 1204. A first width 2510, definedalong the azimuth direction 101, of the high frequency sub-element 1004is similar to a second width 2512 of the low frequency sub-element 1204.As such the multi-frequency elements 2504 may each be formed of 50% ofthe high frequency sub-element 1004 and 50% of the low frequencysub-element 1204 and have a resonance frequency mid-way between that ofthe high frequency sub-element 1004 and the low frequency sub-element1204. In other examples, however, the widths of the sub-elements may bevaried, e.g., not equal, and may be non-uniform throughout an acousticstack, resulting in a range of resonance frequencies. Variations insub-element widths are depicted in FIGS. 31-34 and described furtherbelow.

The plurality of kerfs 2503 may be filled with an electricallyinsulating material, thereby insulating each of the plurality ofelements 2501 from adjacent elements. However, in other examples theplurality of kerfs 2503 may be maintained as air-filled spaces (e.g.,not filled with any additional materials), which may similarly provideelectrical insulation. Furthermore, maintaining the plurality of kerfs2503 as spaces may reduce an overall amount of material of thetransducer array and reduce a weight of the array. The filled pluralityof kerfs 2503 are depicted in FIG. 27 in a first view 2700 along theelevation direction 103 and in FIG. 28 in a second view 2800 along theazimuth direction 101. The fourth acoustic stack 1902 is shownincorporated in a wafer 2702 in FIGS. 27 and 28 .

In addition to filling the plurality of kerfs 2503, a portion of thefront side 2502 of the fourth acoustic stack 1902 may be mechanicallyremoved, similar to the grinding of the back side 2102, to furtherenable ground recovery. The front side 2502 of the fourth acoustic stack1902 may provide electrical grounding. A height 2508 of the portion ofthe fourth acoustic stack 1902 that is removed from the front side 2502is shown in FIGS. 25 and 26 . The amount ground away from the front side2502 of the fourth acoustic stack 1902 may remove portions of the firstlayer of adhesive 1404 parallel with the azimuth direction 101, as shownin FIG. 25 . Grinding of the front side 2502 may also contribute toground recovery in the azimuth and elevation directions by enablingelectrical continuity between the plurality of elements 2501 and anelectrically conductive layer coupled to the front side 2502 of thefourth acoustic stack 1902, described further below.

Returning to FIGS. 27 and 28 , a matching layer block 2704 is laminatedto the front side 2502 of the fourth acoustic stack 1902 after grinding.Although not depicted in FIGS. 27 and 28 , in some examples, aconductive layer, such as the sputtered layer 2302, may be sputteredonto the ground front side 2502 of the fourth acoustic stack 1902 beforecoupling the matching layer block 2704 to the front side 2502. Thematching layer block 2704 may be laminated using a conductive adhesiveand may be a same or different material as the matching layers 1002,1202 of the first comb structure 1100 and the second comb structure1300, respectively. For example, the material of the matching layerblock 2704 may be a gold-coated material, flex conductive materials suchas a spring mass structure, etc. The matching layer block 2704 may beformed of more than layer and may be formed of an electricallyconductive material or a non-conductive material.

The matching layer block 2704 provides a common matching layer to eachtransducer of the fourth acoustic stack 1902, each transducer includingone of the plurality of elements 2501 and defined along the transversedirection 105 by the plurality of kerfs 2503, filled with thenon-conductive material. In other words, the matching layer block 2704is a continuous layer that extends entirely across the front side 2502of the fourth acoustic stack 1902. Similarly, a backing layer block 2706may be coupled to the back side 2102 of the fourth acoustic stack 1902,and connected to each transducer of the fourth acoustic stack 1902 toprovide a common backing layer for each transducer. The backing layerblock 2706 may also be a continuous layer that extends entirely acrossthe back side 2102 of the fourth acoustic stack 1902.

A backing layer block 2706 may be laminated to the back side 2102 of thefourth acoustic stack 1902 to form the wafer 2702, also using aconductive adhesive. The backing layer block 2706 may be formed of oneor more layers, laminated in a stack along the transverse direction 105,and may provide an electrical path to enable application of a voltage tothe plurality of elements 2501. In some examples, the backing layerblock 2706 may include an application specific integrated circuit(ASIC), a flex conductive material, a printed circuit board (PCB), ametal block, etc. In other examples, a backing of some type may becoupled to the back side 2102 of the fourth acoustic stack 1902 insteadof the backing layer block 2706. For example, the backing may be aninterposer connecting a flex circuit to the acoustic stack.

In addition, other examples may include the acoustic stack configuredwith non-continuous matching and backing layer blocks that do not extendcontinuously across each transducers. For example, a plurality ofsmaller matching and backing layer blocks may be coupled to a transducerarray, each block attached to one transducer. Alternatively, thematching and backing layer blocks may cover a few transducers, such astwo or three adjacent transducers, coupling to the acoustic stack insegments.

The wafer 2702 may then be singulated, e.g., singularized, to formindividual transducer arrays 2902, as shown in FIG. 29 in a first view2900 along the elevation direction 103 and in FIG. 30 in a second view3000 along the azimuth direction 101. Each of the transducer arrays 2902includes an array of the plurality of elements 2501, each of theplurality of elements 2501 included in an individual integrated circuit.Singulation may include various methods of die singulation, includingconventional dicing, laser dicing, scribe and break, and dice beforegrind. Each of the transducer arrays 2902 are therefore spaced away fromneighboring transducer arrays as a result of singulation and eachtransducer may be installed in a transducer probe.

Although a width 2904, defined along the azimuth direction 101, of eachof sub-elements (e.g., the high frequency sub-element 1004 and the lowfrequency sub-elements 1204), a width 2906 of each of the plurality ofelements 2501, as well as a width 2908 of each of the transducer arrays2902 is depicted to be uniform in FIG. 29 , the dicing of each of theabove components of the wafer 2702 may be modified to producenon-uniform widths. Variations in widths of the sub-elements, of theplurality of elements, and of the transducers are shown in FIGS. 31-35 .

In a first example 3100 of a non-uniform transducer array 3101, a width3102 of a plurality of elements 3104 may be uniform along the azimuthdirection 101. The plurality of elements 3104 may each include a firstsub-element 3106 and a second sub-element 3108. A first comb structuremay be diced to form the first sub-element 3106 with a width 3110 thatis greater than a width 3112 of the second sub-element 3108. In otherwords the first comb structure may be diced to form wider sub-elements(e.g., the first sub-elements 3106) than dicing of a second combstructure to form the second sub-element 3108.

Alternatively, as shown in a second example 3200 of the non-uniformtransducer array 3103, the first comb structure may be diced so that thefirst sub-element 3106 has a narrower width 3202 than a width 3404 ofthe second sub-element 3108, formed by dicing of the second combstructure. The width 3104 of each of the plurality of elements 3104 maybe uniform along the azimuth direction 101 and the widths of each of thefirst sub-element 3106 and of the second sub-element 3108 may be similarin each of the plurality of elements 3104. Thus the widths of thesub-elements may be readily varied based on dicing of the combstructures.

Furthermore, the comb structures may be diced so that each of thesub-elements have non-uniform widths, as shown in a third example 3300of the non-uniform transducer array 3101. A first element 3302 may beformed of a first sub-element 3304 with a width 3306 that is similar toa width 3308 of a second sub-element 3310. However, in a second element3312, adjacent to the first element 3302, a width 3314 of the firstsub-element 3304 is greater than a width 3316 of the second sub-element3310. A width 3318 of the first element 3302 may be similar to a width3320 of the second element 3312. In other examples, the non-uniformtransducer array 3103 may include elements where the width of the secondsub-element 3310 is greater than the width of the first sub-element3304.

Additionally or alternatively, dicing of an acoustic stack formed bycombining the first and second comb structures, e.g., the third acousticstack 1402 of FIGS. 14-17 , may be modified to vary a width of theplurality of elements. For example, as shown in a fourth example 3400 ofthe non-uniform transducer array 3101, a width 3402 of a first element3404 may be greater than a width 3406 of a second element 3408. Widthsof a first sub-element 3410 and a second sub-element 3412 of each of theelements may be similar, as shown in the first element 3404, ordifferent, as shown in the second element 3408.

Furthermore, singulation of a wafer into individual transducer arraysmay be adjusted to form transducers of varying widths. As illustrated inFIG. 35 , a first transducer array 3502 may be diced to have a firstwidth 3504. A second transducer array 3506 may be diced to have a secondwidth 3508 that is wider than the first width 3504 of the firsttransducer array 3502. Other transducer arrays formed from the samewafer as the first and second transducer arrays 3502, 3506 may havewidths similar to either the first transducer array 3502 or the secondtransducer array 3506 or widths that are different from either of thefirst and second transducer arrays 3502, 3506. Widths of a firstsub-element 3510 and a second sub-element 3512 incorporated in each ofthe elements of each transducer array may be similar to one another ordifferent and may be uniform or non-uniform through the transducerarray.

Electrical leads may be coupled to the matching layer block and thebacking layer block of the transducers array before or aftersingulation. For example, positive electrodes may be coupled to thematching layer block and ground electrodes may be coupled to the backinglayer block. Alternatively, the positive electrodes may be coupled tothe backing layer block and the ground electrodes may be coupled to thematching layer block. Formation of individual circuits with eachtransducer is thereby completed by coupling the transducers arrays toelectrical leads.

In this way, a manufacturing method for multi-frequency transducers maybe fabricated to produce multi-frequency elements and transducer arraysas shown in FIGS. 2 and 4-36 . It will be appreciated that while FIGS.31-35 depict views of the transducers(s) along the elevation direction103 and describe variations in widths of the transducer(s), elements,and sub-elements relative to the elevation direction 103, similarvariations may be applied along the azimuth direction 101. For example,dicing of the comb structures may be modified along the azimuthdirection 101 to provide sub-elements with varying depths (e.g., athickness of the sub-elements along the azimuth direction 101). The combstructures may be combined so that the sub-elements are arrangedadjacent to one another along the azimuth direction 101, instead of orin addition to arrangement of the sub-elements next to one another alongthe elevation direction 103.

A combined comb structure may be diced to generate elements of varyingdepths along the azimuth direction 101 while also varying widths of theelements along the elevation direction 103. Alternatively, the widths ofthe elements may be maintained uniform along the elevation direction 103and varied along the azimuth direction 101. Furthermore, depths of thesingulated transducers may similarly be varied along the azimuthdirection 101 in addition to or instead of along the elevation direction103.

By enabling dimensions of the transducers, elements, and sub-elements tobe varied along both the azimuth direction 101 and elevation direction103, scalable fabrication of the transducers is enabled. A variety oftransducers with different and broad bandwidths and spatial frequencydistribution may be produced from a single wafer. Electrical circuitsare coupled to the wafer prior to singulation, increasing an efficiencyof manufacturing. A quantity of interconnects and control signals in amulti-frequency transducer probe may be similar to a quantity used in asingle-frequency transducer probe. Thus, implementation ofmulti-frequency transducer arrays does not introduce additionalcomplexity to transducer probes.

Ground recovery along both the elevation and azimuth directions allowsgreater flexibility in packaging of a transducer array in a probe. Forexample, a transducer array with a reduced footprint in the elevationdirection may be desirable. In conventional methods, ground recovery maybe difficult when the transducer array is shortened along the elevationdirection. The fabrication process described above with reference toFIGS. 10-38 and 40-42 , however, allows for ground recovery along theazimuth direction instead. Additionally, an apodization functionprovided by the spatial frequency distribution of the broad bandwidthtransducers may allow a transducer probe to be used for more than oneapplication. For example, a single transducer probe may be used for boththerapy and imaging. Image quality may be optimized in both near and farfields due to an enhanced beam focus profile enabled by the apodizationfunction. A footprint of the transducer probe may be optimized for moreefficient packaging. As well, a manufacturing process of an acousticstack of the transducer probe, as illustrated in FIGS. 10-36 may providea universal architecture, based on a collective wafer approach,applicable to all transducer portfolios.

An example of a first routine 3700 for fabricating a multi-frequencyacoustic stack for a transducer probe is depicted in FIG. 37 . A secondroutine 3800, as shown in FIG. 38 , is an example of a routine forforming multi-frequency elements which may be included in the firstroutine 3700. The first and second routines 3700, 3800 describe aprocess similar to the manufacturing process illustrated in FIGS. 10-36. Turning now to FIG. 37 , at 3702, the first routine 3700 includesforming a first acoustic stack, as shown in the second routine 3800 inFIG. 38 .

At 3802 of FIG. 38 , the second routine 3800 includes forming a firstcomb structure, such as the first comb structure 1100 of FIG. 11 , whichhas a first sub-element. The first comb structure may be formed bydicing a first acoustic stack, such as the first acoustic stack 1000 ofFIG. 10 . A second comb structure is formed at 3804, such as the secondcomb structure 1300 of FIG. 13 , which has a second sub-element with adifferent resonance frequency than the first sub-element. The secondcomb structure may be formed by dicing a second acoustic stack, such asthe second acoustic stack 1200 of FIG. 12 . Both the first acousticstack and the second acoustic stack may each include a matching layer, asub-element layer, an optional dematching layer, and a backing layer,the layers stacked along a transverse direction perpendicular to both anazimuth direction and an elevation direction.

The first acoustic stack and the second acoustic stack may be dicedalong opposite directions from one another to impart the first andsecond comb structures with complementary fins. For example, as shown inFIG. 11 , the first acoustic stack may be diced downwards from a topsurface of the first acoustic stack and, as shown in FIG. 13 , thesecond acoustic stack may be diced upwards from a bottom surface of thesecond acoustic stack. Alternatively, the first acoustic stack may bediced upwards from a bottom surface while the second acoustic stack maybe diced downwards from a top surface.

At 3806, the first comb structure is combined with the second combstructure to form an interdigitated combined stack, such as the thirdacoustic stack 1402 of FIG. 14 . The combined stack may be laminated toadhere the comb structures to one another.

At 3808, the routine includes determining if an additional sub-elementis to be incorporated into the combined stack. If no additionalsub-element is to be included, the routine continues to 3704 of thefirst routine 3700 of FIG. 37 . If at least one additional sub-elementis to be incorporated, an additional comb structure is formed at 3810.The additional comb structure may be diced similar to the first combstructure 1100 of FIG. 11 or the second comb structure 1300 of FIG. 13while the combined stack may be diced at 3812 in an opposite manner tocomplement a geometry of the diced additional comb structure. Theadditional comb structure may have a third sub-element with a differentresonance frequency than the first or second sub-elements.

In some examples, the additional comb structure may be a combined combstructure, formed via a similar process as described in 3802-3806 of thesecond routine 3800, so that the additional comb structure has a fourthsub-element in addition to the third sub-element. The additional combstructure with both the third and fourth sub-elements may be similarlydiced to have a complementary geometry to the diced combined stack.

At 3814, the second routine 3800 includes combining the diced combinedstack with the additional comb structure to form a new combined stackwhich may be laminated to adhere the combined stack and the additionalcomb structure to one another. During lamination, a layer ofnon-conductive adhesive may be disposed between the first and secondcomb structures. The method returns to 3808 to again determine if anadditional sub-element is to be incorporated into the (new) combinedstack.

Returning to FIG. 37 , at 3704 of the first routine 3700, the firstacoustic stack, e.g., the combined stack formed via the second routine3800, is diced. Dicing of the first acoustic stack forms a plurality offins separated by a plurality of kerfs in the first acoustic stack. At3706, a base package formed of a conductive material, such as the basepackage 1602 of FIG. 16 , is diced to have fins with a similar geometryto the plurality of kerfs in the first acoustic stack and kerfs with asimilar geometry to the plurality of fins in the first acoustic stack.

The base package and the first acoustic stack are coupled to one anotherand laminated at 3708 to form a second acoustic stack. At 3710, aportion of a back side of second acoustic stack may be removed bygrinding to provide ground recovery. For example, a portion of athickness of a backing layer of the first acoustic stack, as well asportions of the non-conductive adhesive used to laminate the firstacoustic stack, may be removed. By removing a part of the backing layerand the portions of the non-conductive adhesive, ground recovery alongthe azimuth and elevation directions may be enabled.

At 3712, a conductive layer is sputtered on the ground back side of thesecond acoustic stack. The conductive layer may allow electricalconnections to be coupled to the back side of the second acoustic stack,each of the electrical connections included in an integrated circuit ofa final transducer array formed via processing of the second acousticstack. The second acoustic stack is diced at 3714 and kerfs formed bydicing may be filled with a non-conductive material, therebyelectrically insulating each integrated circuit, or transducer, of thesecond acoustic stack from adjacent integrated circuits.

A front side of the second acoustic stack is ground at 3716. The frontside is opposite of the back side and a portion of a thickness of thefront side may be removed by grinding. For example, a matching layer ofthe second acoustic stack may be partially removed. At 3718, a matchinglayer block and a backing layer block may be coupled to the front andback sides, respectively, of the second acoustic to further enableground recovery in the azimuth and elevation directions. At 3720, thefirst routine 3700 includes singulating the second acoustic stack todivide the second acoustic stack into separate transducer arrays. Thetransducer arrays may each be implemented in a transducer probe. Thefirst routine 3700 ends.

In this way, a multi-element transducer array may be provided for atransducer probe. The multi-element transducer array may includesub-elements with different resonance frequencies, distributed along theazimuth and elevation directions in a homogeneous pattern.Alternatively, the sub-elements may positioned to provide varyingspatial frequency distributions along at least one of the azimuth andelevations directions. Frequency apodization and agility is enabledalong the elevation direction and on different structures, e.g., 1D,1.5D, 2D, etc, enabling spatial frequency distribution in complexstructures. Furthermore, frequency apodization and agility is achievedat low cost by fabricating the multi-element transducer through a waferscale approach. A processing of an acoustic stack during the wafer scaleapproach may result in a large distribution of frequency content over atransducer aperture that allows one transducer probe to be use formultiple applications. Image quality may be optimized in both a near andfar field due to a beam focus profile enabled by frequency apodization.The multi-element transducer array may be packaged more efficientlywithin the transducer probe due to ground recovery in both the azimuthand elevation directions and may be used in a variety of transducerportfolios.

The technical effect of fabricating the transducer array via the waferscale approach is that a broad bandwidth transducer array is producedvia a cost efficient method. Another technical effect is that frequencyapodization and agility is enabled along the azimuth and elevationdirections.

As used herein, an element or step recited in the singular and proceededwith the word “a” or “an” should be understood as not excluding pluralof said elements or steps, unless such exclusion is explicitly stated.Furthermore, references to “one embodiment” of the present invention arenot intended to be interpreted as excluding the existence of additionalembodiments that also incorporate the recited features. Moreover, unlessexplicitly stated to the contrary, embodiments “comprising,”“including,” or “having” an element or a plurality of elements having aparticular property may include additional such elements not having thatproperty. The terms “including” and “in which” are used as theplain-language equivalents of the respective terms “comprising” and“wherein.” Moreover, the terms “first,” “second,” and “third,” etc. areused merely as labels, and are not intended to impose numericalrequirements or a particular positional order on their objects.

In one embodiment, a method includes forming a first comb structure witha first sub-element having a first resonance frequency, forming a secondcomb structure, complementary in geometry to the first comb structurewith a second sub-element having a second resonance frequency, combiningthe first and second comb structures to form an interdigitatedstructure, forming a third acoustic stack by coupling the interdigitatedstructure to a base package, and coupling the third acoustic stack to amatching layer block and a backing layer block to form a plurality ofmulti-frequency transducers. In a first example of the method, formingthe first comb structure from a first acoustic stack, the first acousticstack having a first matching layer, the first sub-element, and a firstbacking layer and forming the second comb structure from a secondacoustic stack, the second acoustic stack having a second matchinglayer, the second sub-element, and a second backing layer and whereinthe first and second matching layers include one or more layersconfigured to be electrically conductive along a vertical axis of thefirst and second acoustic stacks. A second example of the methodoptionally includes the first example, and further includes, whereinforming the first comb structure includes dicing a first set of kerfsinto the first acoustic stack to form a first set of fins, the first setof kerfs extending downwards from a top surface of the first acousticstack and wherein forming the second comb structure includes dicing asecond set of kerfs into the second acoustic stack to form a second setof fins, the second set of kerfs extending upwards from a bottom surfaceof the first acoustic stack. A third example of the method optionallyincludes one or more of the first and second examples, and furtherincludes, wherein forming the first and second comb structures furtherincludes forming the first set of fins of the first comb structure withdimensions to allow the first set of fins to be inserted into the secondset of kerfs of the second comb structure and forming the second set offins of the second comb structure with dimensions to allow the secondset of fins to be inserted into the first set of kerfs of the first combstructure. A fourth example of the method optionally includes one ormore of the first through third examples, and further includes, whereindicing the first set of kerfs into the first acoustic stack and dicingthe second set of kerfs into the second acoustic stack includesconfiguring the first and second sets of kerfs with a non-uniformdimension along at least one of an azimuth and an elevation direction. Afifth example of the method optionally includes one or more of the firstthrough fourth examples, and further includes, wherein forming the firstand second acoustic stacks includes dicing the acoustic stacks so thatat least one of the first set of kerfs and the second set of kerfs arenon-uniformly spaced apart along at least one of an azimuth and anelevation direction. A sixth example of the method optionally includesone or more of the first through fifth examples, and further includesdicing the interdigitated structure prior to coupling the interdigitatedstructure to the base package and coupling the diced interdigitatedstructure to a third comb structure. A seventh example of the methodoptionally includes one or more of the first through sixth examples, andfurther includes, wherein coupling the diced interdigitated structure tothe third comb structure incorporates at least one additionalsub-element with a different resonance frequency from either of thefirst and second sub-elements into the interdigitated structure. Aneighth example of the method optionally includes one or more of thefirst through seventh examples, and further includes dicing theinterdigitated structure a number times equal to a number of additionalsub-elements to be incorporated into the interdigitated structure. Aninth example of the method optionally includes one or more of the firstthrough eighth examples, and further includes, wherein dicing theinterdigitated structure includes cutting kerfs into the interdigitatedstructure that are uniformly spaced apart and wherein the third combstructure also has uniformly spaced apart kerfs and a geometrycomplementary to a geometry of the diced interdigitated structure. Atenth example of the method optionally includes one or more of the firstthrough ninth examples, and further includes, wherein dicing theinterdigitated structure includes cutting kerfs into the interdigitatedstructure that are non-uniformly spaced apart and wherein the third combstructure also has non-uniformly spaced apart kerfs and a geometrycomplementary to a geometry of the diced interdigitated structure.

In another embodiment, a method includes combining two or more combstructures into a single acoustic stack to incorporate two or moresub-elements with different resonance frequencies, separating the singleacoustic stack into transducer arrays, each of the transducer arraysincluding at least one transducer, and varying a distribution of the twoor more sub-elements along at least one of an azimuth and an elevationdirection to have frequency apodization and frequency agility. In afirst example of the method, separating the single acoustic stack intotransducer arrays includes singularizing the single acoustic stack todivide the single stack into the transducer arrays. A second example ofthe method optionally includes the first example, and further includesproviding a common ground for the transducer arrays by coupling theacoustic stack to a base package electrically coupled to each of thetransducers and wherein each of the transducers includes a piezoelectricelement formed from the two or more sub-elements. A third example of themethod optionally includes one or more of the first and second examples,and further includes forming an individual integrated circuit with eachpiezoelectric element. A fourth example of the method optionallyincludes one or more of the first through third examples, and furtherincludes, wherein varying the distribution of the two or moresub-elements includes incorporating at least one of the two or moresub-elements into each transducer. A fifth example of the methodoptionally includes one or more of the first through fourth examples,and further includes, wherein varying the distribution of the two ormore sub-elements includes varying relative proportions of the two ormore sub-elements in each transducer. A sixth example of the methodoptionally includes one or more of the first through fifth examples, andfurther includes, wherein separating the single acoustic stack intotransducer arrays includes forming transducer arrays with any of alinear, a curved linear, and a matrix-like arrangement.

In yet another embodiment, a transducer array includes a plurality oftransducers, each transducer incorporated into an individual electroniccircuit and including a piezoelectric element formed from one or moresub-elements and wherein a distribution of the one or more sub-elementsamongst the plurality of transducers is varied along at least one of anazimuth and an elevation direction. In a first example of the transducerarray, at least one of the one or more sub-elements has a differentresonance frequency and wherein the one or more sub-elements areincorporated into each transducer via a wafer scale approach.

This written description uses examples to disclose the invention,including the best mode, and also to enable a person of ordinary skillin the relevant art to practice the invention, including making andusing any devices or systems and performing any incorporated methods.The patentable scope of the invention is defined by the claims, and mayinclude other examples that occur to those of ordinary skill in the art.Such other examples are intended to be within the scope of the claims ifthey have structural elements that do not differ from the literallanguage of the claims, or if they include equivalent structuralelements with insubstantial differences from the literal languages ofthe claims.

The invention claimed is:
 1. A multi-frequency transducer array,comprising: a plurality of piezoelectric elements, a piezoelectricelement of the plurality formed from at least two sub-elements inface-sharing contact, the at least two sub-elements having differentresonance frequencies, and wherein a distribution of the at least twosub-elements is varied along at least one of an azimuth and an elevationdirection.
 2. The multi-frequency transducer array of claim 1, whereinthe piezoelectric elements are a PZT type, and wherein the distributionincludes at least one piezoelectric element with more than 50% of afirst sub-element type and less than 50% of a second sub-element type,50% of the first sub-element type and 50% of the second sub-elementtype, and less than 50% of the first sub-element type and more than 50%of the second sub-element type.
 3. The multi-frequency transducer arrayof claim 2, wherein a first comb structure of a first sub-element has afirst resonance frequency; a second comb structure complementary ingeometry to the first comb structure with a second sub-element has asecond resonance frequency, wherein the first and second comb structuresare combined to form an interdigitated structure.
 4. The multi-frequencytransducer array of claim 3, wherein the first comb structure is formedfrom a first acoustic stack, the first acoustic stack having a firstmatching layer, the first sub-element, and a first backing layer, andthe second comb structure is formed from a second acoustic stack, thesecond acoustic stack having a second matching layer, the secondsub-element, and a second backing layer, and wherein the first andsecond matching layers include one or more layers configured to beelectrically conductive along a vertical axis of the first and secondacoustic stacks.
 5. The multi-frequency transducer array of claim 4,wherein a first set of kerfs is diced into the first acoustic stack toform a first set of fins, the first set of kerfs extending downwardsfrom a top surface of the first acoustic stack and a second set of kerfsis diced into the second acoustic stack to form a second set of fins,the second set of kerfs extending upwards from a bottom surface of thefirst acoustic stack.
 6. The multi-frequency transducer array of claim3, wherein the interdigitated structure is diced prior to coupling theinterdigitated structure to the base package, and the dicedinterdigitated structure is coupled to a third comb structure.
 7. Themulti-frequency transducer array of claim 1, wherein at least onesub-element is single crystal.